1. Field of the Invention
This invention relates to an improvement in a see-through type finger touch sensing device which is provided on a display screen and is used for inputting the data designated by the area touched on the screen by a finger into a computer system.
2. Description of the Prior Art
A see-through type touch keyboard is well known as a man-machine interface unit which is mounted on the display screen of display unit and is used for inputting the data designated by the area touched on the screen into a computer system. For a touch keyboard of this kind, it is important to accurately detect the touch condition. Various systems have been proposed to achieve the required accuracy. One such sensing system measures the change of capacitance between touch sensing electrode and ground induced by a finger touch utilizing the resonant phenomenon of a resonant circuit. This method is explained in detail in the U.S. patent application Ser. No. 437,220 filed Oct. 28, 1982 assigned to the same assignee as this invention.
The touch sensing system utilizing this resonant circuit is advantageous because a touch condition can be detected with a large ON-OFF ratio but, has disadvantage that errorneous operation is caused by contamination of the touch panel surface. Moreover, when a large scale touch panel is used, the capacitance between adjacent electrodes increases and thereby detection sensitivity is decreased.